I'm testing (writing alot of data) MongoDB (1.4.4 and 1.5.5) on Win 2008 R2 and have this issue:
During "new extent allocations" i see the following error: "FlushViewOfFile failed 33".
In almost all cases this is not a problem (except several seconds delay in writing)/ But some times it leads to client disconnects (all clients in the same time with identical error).

The error is (on the client side):

Unable to write data to the transport connection: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond.
at System.Net.Sockets.NetworkStream.Write(Byte[] buffer, Int32 offset, Int32 size)
at Norm.Connection.Write(Byte[] bytes, Int32 start, Int32 size)
at Norm.Protocol.Messages.InsertMessage`1.Execute()
at Norm.Collections.MongoCollection`1.Insert(IEnumerable`1 documentsToInsert
...

The MongoDB is still working and i can connect to it again. But, with the growth of written data these disconnects happens more frequently.
Writing data in 2 threads (1 doc is 7 attributes and about 1 kB in size) fails:
1. First time - after 15 millions of docs written.
2. Second - after 5 millions,
3. Third - after 2 millions.
At the third time database is about 50Gb.

And finally? i've pressed Ctrl+C and find at the log these errors:

FlushFileBuffers failed 6 file: /data/db/fortest.16
FlushFileBuffers failed 6 file: /data/db/fortest.17
FlushFileBuffers failed 6 file: /data/db/fortest.18
FlushFileBuffers failed 6 file: /data/db/fortest.19
FlushFileBuffers failed 6 file: /data/db/fortest.20
FlushFileBuffers failed 6 file: /data/db/fortest.21
FlushFileBuffers failed 6 file: /data/db/fortest.22
FlushFileBuffers failed 6 file: /data/db/fortest.23
FlushFileBuffers failed 6 file: /data/db/fortest.24
FlushViewOfFile failed 487 file: /data/db/fortest.26
FlushFileBuffers failed 6 file: /data/db/fortest.26
FlushViewOfFile failed 487 file: /data/db/fortest.0
FlushFileBuffers failed 6 file: /data/db/fortest.0
FlushViewOfFile failed 487 file: /data/db/fortest.1
FlushFileBuffers failed 6 file: /data/db/fortest.1
FlushViewOfFile failed 487 file: /data/db/fortest.2
FlushFileBuffers failed 6 file: /data/db/fortest.2
FlushFileBuffers failed 6 file: /data/db/fortest.3
---------

As you see, client library is NoRM,
Mongo version is 1.4.4 64 bit (1.5.5 shows the same behavior)

Mongo starts with this command: mongod -v --cpu --syncdelay 1 -logpath e:\logs\Mongod.txt
The log file is in attachment (i've cut off the middle part).

Clients stop write with last "insert fortest.posts" in the log and get the error in a minute.